This invention relates to an improved method and apparatus for separating a molten metal from non- or weakly-magnetizable floating material. The invention has special applications to separating slag from molten steel or iron. In its most basic form the invention process entails inductively pumping a molten metal up from beneath a non- or weakly-magnetizable floating material, such as a slag, and up an incline away from the floating material.
One method used quite frequently in making alloy or carbon steels is the electric-arc process. Although the process suffers the liability of requiring relatively high purity scrap or pig iron, it possesses the advantages of low investment cost, flexibility in the wide range of steel composition which may be produced, and the capability of using only a portion of the capacity of the involved vessels.
Although many variations in the configuration of arc-furnaces exist, one of the most common is a modification of the arc furnace first constructed and operated by Sir William Siemens in the late 1870's. The furnace itself is usually cylindrical and has a shallow hemispherical bottom portion in which the molten metal ultimately resides. The entire furnace is lined with a refractory which is not susceptible to attack by the particular refining process which is to be carried out within. For instance, a furnace used for the basic process will often be lined with magnesite or, in appropriate areas, alumina. Furnaces for use with the acid process may use silica brick. Induction stirrers are sometimes installed beneath the furnace bottom to stir the molten metal and therefore attain a homogeneous product. The upper end of the furnace typically has three large electrodes which extend down to a point near the level of the metal charge. Three-phase electric current is applied to these electrodes. The current flow path is between the three electrodes through the molten metal bath. The cylindrical side walls have one or two doors which can be opened to either introduce the metal charge, slagging or alloying material into the furnace or to allow a human operator to rake slag from the surface of the molten metal. The furnace will also have a small tap-hole opening through the furnace wall with a long open runner on the outside of the wall which is used to decant molten metal from the furnace. There are also provision for tipping the whole arc furnace to decant the molten metal through the tap-hole, in a process step called "tapping", into a ladle for transportation and pouring elsewhere. It should be noted that the tap-hole is placed in a position on the furnace wall which, when the furnace is tapped, is below the surface of the molten metal and its floating slag. The intent of this placement is to allow production of a molten product which is only metal and has none of the slag included therein. It should be apparent that the purity of the molten metal at the end of the metal pour is highly dependent upon the skill of the person operating the arc-furnace. If that person is slow in observing that slag is exiting the furnace, the ultimate metal product may have slag inclusions within.
The operation of a typical arc-furnace begins with the introduction of the raw materials into the furnace. With the power to the arc electrodes off, a charge consisting of metal scrap, alloying constituents that are not easily oxidized, a recarburizer (often coke) and occasionally iron ore are added to the furnace through a side door. Power is then applied to the electrodes and they are brought down to a level close to the charge so as to form an arc. The charge is melted as quickly as is possible to optimize energy consumption. Because of the geometry of the electrodes with respect to the charge, the charge will quickly have a hole created in the relative center of the furnace. The resultant molten metal however quickly heats the lower lining in the furnace and begins melting the charge from the bottom. This combination of effects results in the charge becoming completely molten.
In the typical two-slag or basic process, a first slag forms from the charge and is continuously produced during the period the charge is melting. Silicon and manganese found in the charge react with oxygen and some iron to form a slag composed of complex manganese and iron silicates. Phosphorus, when present, is also oxidized and finds its way into the slag. Oxygen is sometimes injected to promote these reactions. The temperature of the metal bath is raised during the oxidation to advance the removal of excess carbon, as carbon monoxide, and to permit various inclusions to the metal-slag surface and into the slag. The formed carbon monoxide bubbles out of the metal bath as a "boil".
The first slag produced is normally removed and replaced with a reducing slag although a few variations of the basic arc-furnace process entail retaining the first slag and making it into a reducing slag by addition of appropriate materials. However, in the instance when the first slag is removed, the second slag is formed by adding materials such as silica sand, calcium fluoride (fluorspar), calcium oxide (burnt lime), and powdered coke. Excess phosphorus and sulfur appear in the slag and the oxygen level is reduced. Complex metal carbides, which sink to the metal-slag interface and contribute various alloying metals, may also be added at this point. Ideally, the metal bath is only kept under this slag for as short a time as is possible. When the composition of the bath is chemically correct, the furnace is tapped and the steel runs into a waiting ladle for transport and use elsewhere.
Another process using the arc-furnace which is gaining wide acceptance utilizes two vessels, i.e., the furnace and the ladle, to produce the same product in a manner which is more efficient when viewed from an equipment standpoint. In this two-vessel process, the arc-furnace is charged in the same way as in the basic arc-furnace process. An oxidizing slag is formed containing manganese and iron silicates. Excess carbon is oxidized and forms carbon monoxide in a "boil". However, rather than producing a second reducing slag in the arc-furnace, the molten metal is deslagged and transferred to a special ladle. The ladle is of a design which will allow the metal to be inductively stirred, permits the imposition of a vacuum on the metal, allows reheating the metal by removeable arc electrodes, and is tapped from the bottom during the teeming operation.
This ladle refining process (which has many variations) may include a vacuum degassing step to remove hydrogen and some oxygen, a heating step, and the addition of desulphurizing agents such as Misch metal or lime.
Proper use of the ladle during this later refining step has thus far shown good profitability even taking into account the higher initial capital expenditure for the equipment. The increased homogeneity and quality of the steel product and the increased productivity through the availability of the arc furnace during the period it had been used in the refining step have been the basic reasons. Because of the expense of the desulphurization additives placed in the ladle as described above, the clean separation of slag from the molten metal as it leaves the arc-furnace is especially critical in this process.
Regardless of the process used to make the steel, there exists the overriding requirement that the floating slag be effectively separated from the molten steel product. Slags are necessary and are produced even in the open-hearth and basic oxygen processes.
As was described above, one method commonly used to remove slag from electric-arc furnaces involves the arduous and labor-intensive method of using large rakes to skim the molten slag through doors in the arc-furnace wall. Some slag may remain in the furnace using this technique. The use of a small tap-hole below the metal bath surface has thereafter been relied upon to prevent extraneous slag from entering the ladle. Some metal product will remain in the arc-furnace and will be lost if the operator is cautious. If the operator is not cautious, some slag will end up in the ladle. Inclusions of slag in the product steel are highly detrimental to the product properties.
The invention described herein uses a linear inductor, in its preferred embodiment, to provide an effective skimming action and therefore separate slag from molten steel or iron or other metals or alloys.
The use of an electromagnetic linear inductor to pump molten metals is well known in this art.
Throughout history, gravity has been the prime mover in the transportation of molten metals. Materials which are capable of withstanding the tremendous heat associated with molten metals are not, as a rule, of sufficient strength and structural integrity to be used as pump parts. Inductive pumping, although relatively expensive from a cost and energy standpoint, is the winner by default over other mechanical and electrical methods.
Other examples of the use of inductive pumps for molten metal, none of which are relevant to the invention disclosed herein, are discussed below.
A patent to von Starck, U.S. Pat. No. 3,554,670, issued Jan. 12, 1971 discloses an inductive liquid metal conveyer which is capable of pumping metal up an ascending trough. The disclosure in von Starck indicates that the metal conveying trough operates much in the same way as a standard induction motor. However the motor's inductor is cut open along its longitudinal axis and placed beneath this trough. The inductor is said to therefore comprise a stack of slotted or toothed laminations and an electrical winding which is inserted in the slots of the laminations. The stack of laminations for the polyphase winding of the inductor is divided into a number of poles. The length of the poles in a longitudinal inductive direction is indicated by the pole pitch. The disclosure in von Starck avers that a problem existed in a constant conveying force throughout the length of the trough; specifically, that in the area of the last pole division of the inductor the molten metal has a tendency to stagnate and, effectively, dam itself up. The solution to this problem is said to entail the use of a trough having a smaller angle of ascent in the area of the uppermost pole division. The solution is said to achieve substantial complete constancy of the conveying force and the molten metals' conveying speed over the entire length of the trough. No mention is made by von Starck of slag nor any method of separating it from a molten metal.
Another method of using an inductive pump for molten metal is disclosed in U.S. Pat. No. 3,881,915, to Proler, issued May 6, 1975. The apparatus and methods found in Proler center around the concept of providing a substantially continuous moving molten metal stream and introducing into the molten metal stream a charge of metal or metallic oxide particles which are of similar composition to the molten metal stream and which can be magnetically attracted. A magnetic field is placed beneath the moving molten metal stream to draw those particulates beneath the stream to either reduce the oxide particles or melt the metallic particles. Proler utilizes an inductive pump to provide the molten metal stream within the disclosed apparatus. The invention, in a preferred embodiment, has two or more moving metal streams in association with a number of holding tanks. Molten metal is pumped up an incline from a first tank to a second tank. The metal is pumped up another trough back to another level in the first tank. Metal or metal oxide particles are introduced into the troughs and are drawn beneath the surfaces of the metal stream by electromagnetic forces. This arrangement is said to overcome the problems associated with directly recovering the metallic content found in metallic fines. Small metallic particles have a tendency to float on molten metal and are therefore very slow to dissolve in the melt.
Proler discloses the use of a slag outlet at column 11 beginning at line 20. However, Proler does not disclose the advisability of extending the inductor to the lowest point in the molten bath, nor does it disclose the advantages of using a trough having a semi-circular cross-section nor the effect of decreasing the distance between the inductor and the molten metal towards the upper end of the trough.
U.S. Pat. No. 3,980,284, to Shigihara et al., issued Sept. 14, 1976, discloses what is said to be an improvement in the use of an induction pump in intermittent molding processes. The apparatus described in that patent has a well-insulated and constantly heated container made of a refractory material which encloses a molten metal bath. Metal is removed from that container by an inductive pump drawing metal through the container wall from a point in the container. Shigihara continuously circulates molten metal through the pump during the period between molding steps. Thus, fresh molten metal is provided to the pump interior to keep it at a temperature above the solidus and prevent the metal in the less-insulated pump from freezing up.
Shigihara et al. does not teach a method of skimming slag.
U.S. Pat. No. 4,079.920 to Mischenko et al., issued Mar. 21, 1978, discloses the use of an inductive pump in a furnace. The inductive pump can be used either to stir an aluminum metal bath or deliver the molten metal to another processing step such as a continuous casting machine or injection molding machine. The furnace is constructed with an oblique wall extending from the floor at a shallow angle up to a metal discharging port through the furnace wall. The liquid level metal bath resides part way up the oblique wall. The inductor is placed beneath the oblique wall and, when the current is connected to flow in one direction, molten metal is pumped down the wall. However, since liquids are self-levelling in containers, the metal bath will flow around and tend to keep its level. The overall effect is merely to stir the metal. With the current connected to flow in the other direction, the inductive pump will move the metal up the oblique wall and out the discharge port.
Mischenko et al. does not teach a method of skimming slag.